30.000 photos, dozens of scenarios, two sleuths

19 September 2019 • 4 min reading time

When analysing why a building or other structure has collapsed, the forensic engineers of TNO will not rest until they have solved the whole puzzle. Whether it has 20 or 2000 pieces, by studying both the material and the environment, these engineers are always able to put the puzzle back together.

“Fortunately, not many structures collapse in the Netherlands”’ says TNO forensic engineer Huibert Borsje. “On the other hand, we regularly see damage to structures that has a real impact; sometimes financial, sometimes even disruptive to society. Then we spring into action to find the cause of the damage and assess the possible consequences for the structure. This is generally a five-step process.”

STEP 1: COLLECTING INFORMATION

“TNO stands out from many other parties in the way we collect information about the structure and the damage,” says Borsje. “Where an inspection firm will normally start by gathering a huge amount of information in the hope that it will find the data it needs, we collect information that focusses on the specific problem. We start by carefully analysing the nature of the damage and which precise part of the structure has been affected and how.”

Borsje gives a recent inspection of an underground pipeline as example. “A leak was discovered in the pipeline. The customer asked TNO to conduct an analysis to ascertain whether other parts of the pipeline were susceptible to the same type of leak. The customer assumed that an extensive soil survey would be required, however, simply by analysing the existing damage we were able to ascertain at an early stage that the leak had not been caused by ground movements. This made a costly soil survey unnecessary.”

So why don’t other agencies follow this approach? “If there is structural damage, a structural engineer is called in,” explains Borsje. “However, structural engineers make their analyses by doing calculations, so they think in terms of torque, lateral forces, and so on. This is understandable, because that is what they are good at and the same more or less applies to every expert. At TNO, we have learned to detach our analysis from the standard background data and instead approach the problem specifically from the point of view of the damage.”

STEP 2: INSPECTIONS

The analysts prefer to work on location, where they can examine the actual damage as closely as possible. But sometimes they are only involved in a project after all the materials have been removed, or the site is no longer accessible. In these cases, the analysts have to base their assessment on as many as 30,000 photographs, or on debris and other structural parts.

“We always conduct the assessment with at least two experts,” continues Borsje. “The materials specialist examines the material or structure with their expert eye. They may be a structural engineer or an expert in metal corrosion, and one of TNO’s strengths is that we have these experts in house, so we can deploy their knowledge directly.”

Borsje himself examines the damage as a forensic engineer. “I want to know why the material failed and what the influence of the environment was. Which forces, loads or environmental conditions might have caused the damage? This gives me an idea of the bigger picture. The combination of a materials expert and a forensic engineer works perfectly and it is what allows us to solve the most complex puzzles.”

STEP 3: PREPARING A HYPOTHESIS

Once the researchers have considered all the possible scenarios – and there may be dozens – they prepare a hypothesis. “A more detailed analysis leaves us with a number of more likely scenarios of what caused the damage. In practice, this is the most difficult step in the investigation, because it is very difficult to know if you have actually considered all the possible causes. For example, errors may have been made during construction that simply no one knows about.”

“Take the example of the collapsed parking garage in Eindhoven,” Borsje remembers. “Because we knew that the top floor had collapsed first, we first analysed the debris to pinpoint where that floor had given way. For each fracture surface, we considered the question: suppose it started here; how would the structure have collapsed and does that match the pattern of debris lying on the ground? After we’d examined all the fracture surfaces, only one remained that could have set off the event.”

STEP 4: VERIFICATION

“Once we’ve reached a hypothesis, in the fourth step we verify whether it is correct. We conduct structural calculations to determine whether the structure could indeed have failed as it did. If the answer is negative, we have a problem, because that means we did not do our job properly and have to go back to step 2 or 3. Fortunately that’s never happened to me,” Borsje laughs.

STEP 5: REPORTING

The fifth and final step involves delivering the report. “This too is an art in itself: you have to write up everything you have found in such a way that even non-technical readers understand the successive steps. Alongside secondary goals such as establishing legal liability, the report also serves the primary goal of our work: once you have ascertained the true nature of the problem, you can take measures to prevent it happening again.”

THE BEST JOB IN THE WORLD

“I've got the best job in the world,” says Borsje. “For me, solving problems is the most satisfying thing there is. I can still remember our first meeting with the director of the parking garage in Eindhoven. He anxiously wanted to know if we thought we could find the cause. I replied that whether it would take me one day or a hundred, I was going to find out what happened, no doubt about it.”

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